Teaching aid for audibly and visually illustrating the notes of the staff

ABSTRACT

A blackboard, having one or more staffs and the notes thereof on its front face, has on its rear face a series of electric conductors aligned with the respective notes on the front face, each conductor being connected to an audio oscillator for producing the frequency of the corresponding note. The output of the capacitive pickup head, when the latter is placed on a note, is conducted to a low amplitude suppressor and afterwards to an amplifier, which latter is connected to a loudspeaker. The low amplitude suppressor insures that the tone of an audio oscillator is audibly reproduced, without swelling or dying away, only when the pickup head is very close to a note.

United States Patent Schulz et al.

[ 51 May 9, 1972 [54] TEACHING AID FOR AUDIBLY AND VISUALLY ILLUSTRATING THE NOTES OF THE STAFF [72] Inventors; Hansrichard Schulz; Joachim Bulow;

Berthold Schmitt; Rudolf Schneider, all of Villingen, Germany Firrna Saba Schwarawalder Apparatebauanstalt August Schwer Sohne G.m.b.l-l., Villingen, Germany [22] Filed: Nov. 20, 1970 [21] Appl.No.: 91,248

[73] Assignee:

[30] Foreign Application Priority Data Nov. 26, 1969 Germany ..P 19 59 309.0

[52] U.S.Cl ..84/471 [51] Int. Cl.

[58] Field of Search ..84/470, 471, 477, 478, 483, 84/ 1 .01

[56] References Cited UNITED STATES PATENTS 3,382,750 5/1968 Hiyama ..'....84/47l FOREIGN PATENTS OR APPLICATIONS 84,470 1958 Denmark ..84/470 Primary ExaminerRichard B. Wilkinson Assistant Examiner-John F. Gonzales Attorney-Michael S. Striker [5 7] ABSTRACT A blackboard, having one or more staffs and the notes thereof on its front face, has on its rear face a series of electric conductors aligned with the respective notes on the front face,

each conductor being connected to an audio oscillator for producing the frequency of the corresponding note. The output of the capacitive pickup head, when the latter is placed on a note, is conducted to a low amplitude suppressor and afterwards to an amplifier, which latter is connected to a loudspeaker. The low amplitude suppressor insures that the tone of an audio oscillator is audibly reproduced, without swelling or dying away, only when the pickup head is very close to a note.

21 Claims, 8 Drawing Figures PATENTEDMAY 1 2 Sum 3 0| 5 AGFEDCBAGFEDCBA A GFEDCBAGFEDCBA BACKGROUND OF THE INVENTION The invention relates to a teaching aid for audibly and visually illustrating the notes of the staff.

One such teaching aid of the prior art has a blackboard of electrically insulating material having, on the front face, one or more staffs with their notes and, on the rear face a seriesof conductors aligned with the respective notes, neighboring conductors being separated by a grounded shielding layer. Each conductor is connected with an audio signal generator for producing a frequency corresponding to the tone of the note. A pickup head enables capacitive coupling between the conductors and an amplifier, which is connected to at least one loudspeaker.

This known teaching aid enables audible illustration of the notes of the staff shown on the blackboard by touching the pickup head to each note in turn.

When the pickup head is sufficiently near to a conductor to be capacitively or inductively coupled to it, the respective audio signal generator is turned on by a control circuit. The generator must first begin to oscillate before the tone is reproduced by the loudspeaker. The initiation of oscillation is difficult to control so that there can be overshoot or frequency shift. Consequently, the tone at the very beginning is not clean.

SUMMARY OF THE INVENTION staff, a plurality of audio signal generators for producing the frequency of the notes, the audio signal generators being connected to respective ones of the conductors and being simultaneously in operation, a portable tune pickup head for picking up the signal on a respective one of the conductors when the pickup head is placed in close proximity to the corresponding note of the staff, at least one sound transducer connected to the pickup head for audibly reproducing the signal on the latter, and low amplitude suppression means connected between the sound transducer and the pickup head for preventing conduction to the sound transducer of any signal that lies below a predetermined amplitude, whereby there is no audible reproduction of any signal that lies below said predetermined amplitude.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1a and 1b are block diagrams of the invention;

FIG. 2 is a perspective drawing of the blackboard;

FIG. 3 is a detail view of the control section and loudspeaker baffle of the blackboard;

FIG. 4 is a view in cross-section and on expanded scale of a portion of the rear face of the black-board;

FIG. 5 is a schematic diagram of the circuit of the tone pickup head;

FIG.- 6 is a schematic diagram of the low amplitude suppressor; and

FIG. 7 is a schematic diagram of the audio oscillator of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS pickup head 3 having a capacitive pickup nose 3a and a preamplifier 3b. Thepickup head 3 conducts the capacitively.

coupled oscillations to a .low amplitude suppre'ssor'4, which'is connected to the'input of an audio amplifier Sthat drives two loud speakers'6. If the pickup head 3 is placed on a line or space on the staff of the blackboard, the loudspeaker reproduces the tone corresponding to the note of that line or space.

FIGS. 2 and 3 show the external physical construction of the blackboard of the invention. One end of the black-board 1 comprises a control section 7 and a loudspeaker baffle 8 that are combined into a single unit. The control section 7 houses both the audio signal generators 2, the low amplitude suppresson and the audio amplifier 5. As .best shown in FIG. 3, the front plate of the control section 7 has a row of push buttons 9 that are horizontally aligned with the lines and spaces of the two-staffs on the blackboard. When a button is pressed the tone of the corresponding note is reproduced by the loudspeakers 6.

These push buttons 9 can be pressed one after the other or two or more push buttons can be pressed simultaneously, so that the tone of one note can be heard or the tones of two or more notes can be heard simultaneously. In accordance with the invention, a separate manual, much in the manner of an electronic organ, can be provided for the push buttons 9.

In addition to the push buttons 9, there are also provided on the control section 7 a series of three position switches 10, each of these switches corresponding to a different one of the 12 tones of the scale. These switches 10 enable every tone to be raised or lowered one half step, so that any desired tone or sequence of tones within the range of the device is possible.

An on-off switch 11, a volume control 12 and a pilot lamp 13 are arranged at the upper end of the baffle front face.

FIG. 4 shows the arrangement of the electrical conductors 14 on the rear face of the blackboard 1. A respective conductor l4-is aligned with each note of the on the front face of the blackboard. The three position switches 10 connect these conductors to the respective oscillators, or audio signal generators, 2. Two neighboring conductors 14 are de-coupled by an intermediate metallic, grounded, shielding layer 15. Allof the shielding layers 15 are connected together.

The conductors l4 and shielding layers 15 can consist of metal foil cemented to the rear face of the black-board 1. However, in accordance with the invention, the conductors and shielding layers can be composed of a conductive lacquer that is sprayed, using a stencil, on the rear face of the blackboard.

FIG. 5 schematically shows the preamplifier 3b contained in the tone pickup head 3. The preamplifier comprises a field ef fect transistor T1, the inherently high input impedance of which is made still higher by a bootstrap circuit. To this end, the source resistor R1 is connected by a condenser C1 to the junction between the resistors R2 and R3, which latter com- A resistor R4 connects the gate of the field effect transistor to the pickup nose 3a of the tone pickup head. The output of this preamplifier is connected by conductors e and b of a shielded cable to the low amplitude suppressor 4, the circuit of which is shown in FIG. 6. One coupling member 17 of the cable is shown in FIG. 5, the other coupling member 16 being shown in FIG. 6.

The circuit shown in FIG. 5 is so designed that both the signal and the supply current flow through the conductor e, whereas the conductor b serves only as a ground connection.

The pickup nose 3a capacitively couples the preamplifier 3b to the field surrounding a conductor 14. Since the audio signal generators 2 operate continuously, it is unnecessary for the teacher to switch on individually each generator for the pupils to hear the tone of this or that note. Continuous operation of the audio signal generators also avoids transient start up effects and control elements for turning each generator on and off.

The teaching aid provides a clearer demonstration of the tone of a note if the tone is reproduced onlywhen the tone pickup head touches the note in question, the notes of the scale being reproduced on the front face of the black-board 1. Moreover, the tones should begin and end without annoying crackling. In accordance with the invention, the effective time constant of the low amplitude suppressor 4 which time constant is largely determined by the filter section should be of such a value that the circuit for transmitting the signals of the audio generators is turned on and off substantially without overshoot. A time constant of the order of from about 0.1 to 0.5 seconds is particularly advantageous.

The circuit of the low amplitude suppressor 4, which is shown in FIG. 6, acts to suppress all signals that lie below a predetermined level. Any interference, such as power supply hum, is suppressed. However, the more important effect of the low amplitude suppressor is to insure that the tone begins abruptly and without crackling, when the pickup head 3 is placed against a note, and that the tone stops abruptly and without crackling, when the head 3 is moved from the note. The circuit shown in FIG. 6 has a signal channel composed of the transistors T2 and T3, connected together by the resistors R 6 and R7, and a control circuit having the transistors T4, T5 and T6. The transistor T2, with its base resistor R4, the adjustable emitter resistor R5, and the collector resistor R10, is connected common emitter. Part of the resistor R5 is shunted by a capacitor C2, which is connected to the adjustable tap of the resistor R5 for the purpose of controlling the gain. The output transistor T3 is connected with its emitter resistor R8 common collector so as to increase the input impedance of the output transistor and thereby to decouple the low amplitude suppressor 4 from the following audio amplifier 5, which latter is shown in FIG. 1.

A regulatable resistor means, which in the embodiment illustrated'is the field effect transistor T4, is shunted across the audio signal channel to act as a shorting switch. A capacitor C3 connects the source of the field effect transistor to the junction between the resistors R6 and R7, and a resistor R9 is shunted across the source and drain of the field effect transistor.

The field effect transistor T4 is turned on and off by a control circuit connected in parallel with the audio signal channel. This control circuit comprises an amplifying transistor T5 having an emitter resistor R11, a collector resistor R12 and an emitter capacitor C4; a high pass filter having capacitors C5 and C6 and a resistor R13; a full wave rectifier composed of diodes D5 and D6 and a resistor R14; a filter section having the capacitors C7 and C8 and a resistor R15; and a switching transistor T6 having two collector resistors R16 and R17 that serve as voltage dividers. The junction between these latter two resistors is connected by resistor R18 to the gate of the field efiect transistor T4. A capacitor C9 provides an altemating current short for the gate of the field effect transistor T4. Current for the circuit is provided through the resistor R19, the capacitor C serving to smooth the current.

The circuit just described operates in the following manner. If the amplitude of the audio signal on the conductor e is sufficiently large, the signal passes through the amplifying transistor T2 and the impedance converter T3 on to the audio amplifier 5. From this signal there is also obtained a control voltage, the signal being amplified by transistor T5, rectified (D5 and D6), and filtered (R14, R15, C7, and C8). The resulting control voltage is fed to the field effect transistor T4 by way of the switching transistor T6 and the voltage divider R16, R17. With signals smaller than a predetermined amplitude. the switching transistor T6 remains cut off, so that the voltage at the junction between the resistors R16 and R17 is virtually at the potential of the supply voltage. The field effect transistor, which is connected to this junction by the resistor R18, is turned on, thereby short-circuiting the input transistor T2. If the audio signal is equal to or greater than the predetermined value, the switching transistor T3 is turned on, the field effect transistor T4 thereby being turned off and permitting a signal to pass through the audio signal channel.

The high pass filter C5, C6 and R15 suppresses interference, particularly any undesired effects of the low frequency mains voltage, and prevents motor boating.

The low amplitude suppressor has a second input f, which is directly connected to the base of the output transistor T3. This input can be connected to a pickup matrix that enables direct connection to individual audio signal generators by means of the push button switches 9.

' An important part of the invention is the control section 7, which, aside from the blackboard 1, determines the size of the teaching aid, and the audio signal generators 2 of the control section. These signal generators must produce frequencies that lie within the compass of the normal speaking and singing voice, the range of which (not including overtones) lies between approximately 200 and 1,000 cycles per second. This relatively low frequency range excludes, for reasons of cost and size, LC oscillators. RC oscillators, particularly RC phase shift oscillators are suitable, although their adjustment and their amplitude and frequency stabilization are critical.

In accordance with the invention, a novel RC oscillator is I used with the teaching aid, the oscillator being insensitive to voltage and temperature fluctuations and having a large range of adjustment, which latter feature is extremely advantageous in the manufacture and alignment of the teaching aid.

The audio signal generator of the invention comprises RC transistor oscillators, the output stages of which'are direct current coupled by negative feed-back resistors to the emitters of emitter follower stages. The output stage is common emitter connected. In this way, the working points of the transistors are extremely well stabilized. The aforesaid emitter resistor can have a value that renders the frequency of the oscillator relatively immune to voltage fluctuations, the frequency changing less than 0.01 percent per volt.

A necessary condition for oscillation is that the phase shift between the input and the output is 360. The common emitter stage shifts the phase 180, and the positive feed-back circuit shifts the phase another 180. A further feature of the invention is that the positive feed-back circuit incorporates a series of 3 RC stages, the first causing a phase shift of 30, the second a phase shift of 60, and the third a phase shift of to make a total of of phase shift. This design enables a particularly simple adjustment of the oscillator without stopping the oscillation or appreciably changing the amplitude of the oscillation. The oscillator is adjusted or aligned by varying the resistance of an adjustable resistor in the third RC section.

A further feature of the novel oscillator is a capacitor for providing temperature compensation, the capacitor providing an alternating current shunt path across the collector of the common emitter connected transistor. The temperature coefficient of this capacitor is large with respect to that of the RC sections in the positive feed-back circuit. The temperature coefiicient of the capacitor, which has little effect on the amplitude of the oscillation, is so chosen that the frequency and amplitude of the oscillation is virtually constant within the range of temperatures under consideration. If the capacitors ,of the three RC sections have a temperature coefficient of approximately 130 l0/C. the temperature compensating capacitor advantageously has a temperature coefficient of approximately 330 l0/ C.

The circuit of these audio signal generators 2 is shown in FIG. 7. The circuit comprises a two-stage RC transistor oscillator. The output transistor T8, which is connected common emitter, is, in contradistinction to known RC oscillators, not inductively, but direct current, connected by the negative feed-back emitter resistor R20 to the emitter of transistor T7. A resistor R2] connects the emitter of transistor T8 to ground, and a resistor R22 connects the collector of this transistor to the positive supply voltage. The collector is also directlyconnected to the input of the three RC stages, these stages comprising the resistors and capacitors R23, C11, R24, C12, R25, and C13. In accordance with the invention, the resistance values of these three RC stages are such that the input stage causes a phase shiftof 30, the second stage a phase shift of 60, and the third stage a phase shift of 90, insuring that each stage causes an optimum phase shift for maximum frequency stability.

The negative feed-back emitter resistor R20, which simultaneously couples together the transistors R7 and T8, has such a value that the oscillation frequency is relatively independent of supply voltage, the frequency variation being less than 0.01 percent /volt. The resistor R20 also temperature stabilizes the working points of the transistor stages. Temperature changes caused by the heat dissipating components of the circuit are compensated by a capacitor C14 that is connected to form an alternating current shunt across the output of the transistor T2. The temperature coefficient of this capacitor, which is greater than that of the capacitors C11, C12 and C13, is so chosen that the desired temperature compensation is obtained. In the circuit of the invention, the capacitor C14 advantageously has a temperature coefficient of approximately 330 l0/C., the capacitors C11, C12 and C13 having a temperature coefficient of approximately 130 l0 /C.

The trimming resistor R25 enables adjustment of the oscillator through a relatively wide range without appreciably affecting the amplitude.

in accordance with the invention, 12 audio oscillators of the kind shown in FIG. 7 are provided for the half steps of one octave, three oscillators of successive half steps being selectively connected to a conductor 14 by a three position switch on the control section 7. The half steps lying below this octave can be obtained by a frequency divider that divides the frequencies of the half steps within this octave.

In accordance with the invention, the low amplitude suppressor 4 or the audio amplifier 5 can be connected to other signal sources for obtaining the frequencies of the notes of the scale. These sources can include, among others, broadcast apparatus, phonographs, and tape play-back machines. This versitility of the invention is extremely advantageous when used in schools.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of circuits difiering from the types described above.

While the invention has been illustrated and described as embodied in teaching aid for audibly and visually illustrating the notes of the staff, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

We claim: 1. A teaching aid for audibly and visually illustrating the notes of the staff, comprising, in combination, a black-board having thereon at least one staff, said black-board being made of electrically insulating material; a plurality of electric conductors associated with the rear face of said blackboard, each of said conductors corresponding to a respective one of the notes of said staff and being aligned with the respective note on said staff; a plurality of audio signal generators for producing the frequency of said notes, said audio signal generators being connected to respective ones of said conductors and being simultaneously in operation; a portable tone pickup head for picking up the signal on a respective one of said conductors when said pickup head is placed in close proximity to the corresponding note on said staff; at least one sound transducer connected'to said pickup head for audibly reproducing the signal on the latter; and low amplitude surpressor means connected between said sound transducer and said pickup head for preventing conduction to said sound transducer of any signal that lies below a predetermined amplitude, whereby there is no audible reproduction of any signal that lies below said predetermined amplitude.

2. A teaching aid as defined in claim 1, wherein said low amplitude surpressor means includes regulatable resistor means for providing a short circuit whenever the signal amplitude falls below said predetermined amplitude so as to prevent conduction of a signal to said sound transducer, at all other times the resistance of said regulator resistor means being sufficient to permit conduction of a signal to said sound transducer.

3. A teaching aid as defined in claim 2, wherein said regulatable resistor means is a field effect transistor.

4. A teaching aid as defined in claim 2,

wherein said low amplitude surpressor means has an input transistor connected to receive a signal from said pickup head and, connected to said input transistor, a control circuit for controlling the resistance of said regulatable resistor means, said control circuit including an amplifying transistor connected to said input transistor to receive as input the output thereof, a high pass filter connected to the output of said amplifying transistor, rectifying means connected to rectify the output of said high pass filter, filter means connected to the output of said rectifying means for smoothing the output thereof, and a switching transistorconnected to receive the output of said filter means as input, the output of said switching transistor being connected to said regulatable resistor means.

. 5. A teaching aid as defined in claim 4,

wherein said regulatable resistor means is a field effect transistor, the gate of which is connected to the output of said switching transistor.

6. A teaching aid as defined in claim 4,

wherein said controlcircuit has a time constant, said time constant being largely determined by said filter means and of sufficient length so that conduction and non-conduction of the signal to said sound transducer is obtained without overshoot.

7. A teaching aid as defined in claim 6,

wherein said time constant is between approximately 0.1

and 0.5 seconds. i

8. A teaching aid as defined in claim 1,

wherein said pickup head has a preamplifier, said preamplifier having an input field effect transistor connected in'a bootstrap circuit for raising the input impedance of said field effect transistor; and an amplitude limiting circuit connected in the input of said field effect transistor.

9. A teaching aid as defined in claim 8,

including a voltage divider connected in the input of said field effect transistor and wherein said amplitude limiting circuit comprises two first series connected diodes and two second series connected diodes connected in parallel with said voltage divider, the polarity of said first series connected diodes being opposite to that of said second series connected diodes.

10. A teaching aid as defined in claim 1,

wherein each of said audio signal generators includes an RC transistor oscillator having first and second transistors respectively as input and output transistors, said second transistor being connected common emitter, and said first transistor being connected to said second transistor as an emitter follower; and resistance means for direct current connecting the emitter of said first transistor to the base of said second transistor.

11. A teaching aid as defined in claim 10,

including a positive feed-back circuit for connecting the output of said second transistor to the input of said first transistor, said feed-back circuit including first, second and third RC phase shifting stages, respectively causing a phase shift of 30, 60, and 90, for a total of 180.

12. A teaching aid as defined in claim 11, wherein the resistor of said third RC stage is adjustable in value.

13. A teaching aid as defined in claim 11,

wherein said first, second and third RC stages have a temperature coefficient, and including a temperature compensating capacitor shunted across the output of said second transistor, the temperature coefficient of said capacitor being large with respect to the temperature coefficient of said first, second and third RC stages.

14. A teaching aid as defined in claim 13,

wherein said temperature compensating capacitor has a temperature coefficient of approximately 330 l C., and the capacitors of said RC stages have a temperature coefficient of approximately 130 IO I C.

15. A teaching aid as defined in claim 1, wherein there are twelve said audio frequency generators for producing the frequencies of the half steps of an cetave; and switch means for selectively connecting said generators in groups of three of consecutive half steps to respective ones of said conductors.

16. A teaching aid as defined in claim 15, including frequency dividing means for dividing the frequencies of said audio frequency generators to obtain frequencies of those notes that lie below said octave.

17. A teaching aid as defined in claim 1,

including switch means for selectively connecting said audio frequency generators to said sound transducer.

18. A teaching aid as defined in claim 17,

wherein there are twelve said audio frequency generators for producing the frequencies of the half steps of an octave; and further switch means for selectively connecting said generators in groups of three of consecutive half steps to respective ones of said conductors, said switch means and further switch means being aligned with the notes to which they correspond.

19. A teaching aid as defined in claim 1,

wherein said sound transducer is at least one loudspeaker and baffle means combined with said blackboard for said loudspeaker.

20. A teaching aid as defined in claim 1,

including switch means for connecting said audio signal generators to said sound transducer, and manual means for mounting said switch means.

21. A teaching aid as defined in claim 1,

wherein said conductors are sprayed conductive lacquer. 

1. A teaching aid for audibly and visually illustrating the notes of the staff, comprising, in combination, a black-board having thereon at least one staff, said black-board being made of electrically insulating material; a plurality of electric conductors associated with the rear face of said blackboard, each of said conductors corresponding to a respective one of the notes of said staff and being aligned with the respective note on said staff; a plurality of audio signal generators for producing the frequency of said notes, said audio signal generators being connected to respective ones of said conductors and being simultaneously in operation; a portable tone pickup head for picking up the signal on a respective one of said conductors when said pickup head is placed in close proximity to the corresponding note on said staff; at least one sound transducer connected to said pickup head for audibly reproducing the signal on the latter; and low amplitude surpressor means connected between said sound transducer and said pickup head for preventing conduction to said sound transducer of any signal that lies below a preDetermined amplitude, whereby there is no audible reproduction of any signal that lies below said predetermined amplitude.
 2. A teaching aid as defined in claim 1, wherein said low amplitude surpressor means includes regulatable resistor means for providing a short circuit whenever the signal amplitude falls below said predetermined amplitude so as to prevent conduction of a signal to said sound transducer, at all other times the resistance of said regulator resistor means being sufficient to permit conduction of a signal to said sound transducer.
 3. A teaching aid as defined in claim 2, wherein said regulatable resistor means is a field effect transistor.
 4. A teaching aid as defined in claim 2, wherein said low amplitude surpressor means has an input transistor connected to receive a signal from said pickup head and, connected to said input transistor, a control circuit for controlling the resistance of said regulatable resistor means, said control circuit including an amplifying transistor connected to said input transistor to receive as input the output thereof, a high pass filter connected to the output of said amplifying transistor, rectifying means connected to rectify the output of said high pass filter, filter means connected to the output of said rectifying means for smoothing the output thereof, and a switching transistor connected to receive the output of said filter means as input, the output of said switching transistor being connected to said regulatable resistor means.
 5. A teaching aid as defined in claim 4, wherein said regulatable resistor means is a field effect transistor, the gate of which is connected to the output of said switching transistor.
 6. A teaching aid as defined in claim 4, wherein said control circuit has a time constant, said time constant being largely determined by said filter means and of sufficient length so that conduction and non-conduction of the signal to said sound transducer is obtained without overshoot.
 7. A teaching aid as defined in claim 6, wherein said time constant is between approximately 0.1 and 0.5 seconds.
 8. A teaching aid as defined in claim 1, wherein said pickup head has a preamplifier, said preamplifier having an input field effect transistor connected in a bootstrap circuit for raising the input impedance of said field effect transistor; and an amplitude limiting circuit connected in the input of said field effect transistor.
 9. A teaching aid as defined in claim 8, including a voltage divider connected in the input of said field effect transistor and wherein said amplitude limiting circuit comprises two first series connected diodes and two second series connected diodes connected in parallel with said voltage divider, the polarity of said first series connected diodes being opposite to that of said second series connected diodes.
 10. A teaching aid as defined in claim 1, wherein each of said audio signal generators includes an RC transistor oscillator having first and second transistors respectively as input and output transistors, said second transistor being connected common emitter, and said first transistor being connected to said second transistor as an emitter follower; and resistance means for direct current connecting the emitter of said first transistor to the base of said second transistor.
 11. A teaching aid as defined in claim 10, including a positive feed-back circuit for connecting the output of said second transistor to the input of said first transistor, said feed-back circuit including first, second and third RC phase shifting stages, respectively causing a phase shift of 30*, 60*, and 90*, for a total of 180*.
 12. A teaching aid as defined in claim 11, wherein the resistor of said third RC stage is adjustable in value.
 13. A teaching aid as defined in claim 11, wherein said first, second and third RC stages have a temperature coefficieNt, and including a temperature compensating capacitor shunted across the output of said second transistor, the temperature coefficient of said capacitor being large with respect to the temperature coefficient of said first, second and third RC stages.
 14. A teaching aid as defined in claim 13, wherein said temperature compensating capacitor has a temperature coefficient of approximately + 330 . 10 6/*C., and the capacitors of said RC stages have a temperature coefficient of approximately + 130 . 10 6/* C.
 15. A teaching aid as defined in claim 1, wherein there are twelve said audio frequency generators for producing the frequencies of the half steps of an octave; and switch means for selectively connecting said generators in groups of three of consecutive half steps to respective ones of said conductors.
 16. A teaching aid as defined in claim 15, including frequency dividing means for dividing the frequencies of said audio frequency generators to obtain frequencies of those notes that lie below said octave.
 17. A teaching aid as defined in claim 1, including switch means for selectively connecting said audio frequency generators to said sound transducer.
 18. A teaching aid as defined in claim 17, wherein there are twelve said audio frequency generators for producing the frequencies of the half steps of an octave; and further switch means for selectively connecting said generators in groups of three of consecutive half steps to respective ones of said conductors, said switch means and further switch means being aligned with the notes to which they correspond.
 19. A teaching aid as defined in claim 1, wherein said sound transducer is at least one loudspeaker and baffle means combined with said blackboard for said loudspeaker.
 20. A teaching aid as defined in claim 1, including switch means for connecting said audio signal generators to said sound transducer, and manual means for mounting said switch means.
 21. A teaching aid as defined in claim 1, wherein said conductors are sprayed conductive lacquer. 